The sequence specificity of two resolved pyrimidine dimer specific endonucleases (correndonuclease) which act on thymine-thymine, thymine-cytosine and cytosine-cytosine cyclobutane dimers is to be examined using a UV modified restriction fragment of the lac operon, operator promoter (lac op) region. The two structural genes controlling the synthesis of the two endonucleases are to be cloned and the numbers of gene copies to be amplified for protein structure determinations. The uvrC structural gene is to be cloned to facilitate isolations and characterizations of its gene product.